Crystal Chemistry of Hydrous Calcium Silicates: I, Substitution of Aluminum in Lattice of Tobermorite

Mixtures of 0.8 moles of CaO per mole of SiO 2 plus Al 2 O 3 were prepared from lime, kaolin, and tripoli (microcrystalline quartz); the amounts of SiO 2 to Al 2 O 3 were varied to give from 0.2 to 20.7% Al 2 O 3 by weight of dry solids. After hydrothermal treatment (170° to 175°C.), the products were examined by differential thermal analysis and by X‐ray diffraction. A homogeneous solid identified as the mineral tobermorite (4CaO.5SiO 2 .5H 2 O) and containing up to 4 or 5% Al 2 O 3 was obtained. Increasing the amount of Al 2 O 3 in the raw mixture above about 5% resulted in the formation of the hydrogarnet 3CaO.Al 2 O 3 .SiO 2 .4H 2 O as a second phase. Allowing for the Al 2 O 3 combined in this solid, it was indicated that slightly more Al 2 O 3 was substituted in the tobermorite as the amount was increased in the raw mixture. It is suggested that the Al 3+ ions probably assume tetrahedral coordination when substituting for the Si 4+ ions.